Hydrothermal Synthesis and Characterization of PEG-Mn3O4 Nanocomposite

Here, we report on the synthesis of PEG-Mn3O4 nanocomposite (NP’s) via a hydrothermal route by using Mn(acac)2, ethanol, NH3 and PEG-400. The crystalline phase was identified as Mn3O4. The crystallite size of the PEG-Mn3O4 nanocomposite was calculated as 12±5 nm from X-ray line profile fitting and the average particle size from TEM was obtained as 200 nm. This reveals polycrystalline character of Mn3O4 NP’s. The interaction between PEG-400 and the Mn3O4 NP’s was investigated by FTIR. Temperature independent AC conductivity of PEG-Mn3O4 nanocomposite beyond 20 kHz provides a strong evidence of ionic conduction through the structure. The conductivity and permittivity measurements strongly depend on the secondary thermal transition of nanocomposite beyond 100°C. Above that temperature, Mn3O4 particles may interact with each other yielding a percolated path that will facilitate the conduction. On the other hand, the relatively lower activation energy (Ea=0.172 eV) for relaxation process suggests that polymer segmental motions of PEG and electrons hopping between Mn2+ and Mn3+ may be coupled in the sample below 100°C. Room temperature magnetization curve of the sample does not reach to a saturation, which indicates the superparamagnetic character of the particles. As the temperature increases, the frequency at which (ε″) reaches a maximum shifted towards higher frequencies. The maximum peak was observed at 1.4 kHz for 20°C while the maximum was detected at 23.2 kHz for 90°C.


Introduction
Among magnetic nanoparticles, manganese oxide (Mn 3 O 4 ) as a magnetic transition-metal oxide is an important material. It has a wide range of applications as catalyst, ion-exchange medium, molecular adsorbent, and in electrochemical materials and varistors [1][2][3][4][5]. Furthermore, Mn 3 O 4 has been widely used as the main source of ferrite materials, finding extensive applications in electronics and information technologies. Mn 3 O 4 is known to crystallize in the normal spinel structure with a tetragonal distortion elongated along the c-axis. Manganese ions are located in the tetrahedral A-sites (Mn 2+ ) and octahedral B-sites (Mn 3+ ).
In order to obtain monodisperse nanocomposites, different kinds of stabilizers have been utilized, including polymers, dendrimers, surfactants and other ligands, to overcome the van der Waals interaction between the nanoclusters which otherwise lead to agglomeration [20]. For long-term stability, researchers often employ surfactants as either stabilizers or templates in the synthesis to decrease the surface energy, control the growth and shape of the particles, and act against aggregation [20]. It has been reported that PEG with a uniform and ordered chain structure is easily absorbed at the surface of metal oxide colloid [21]. When the surface of the colloid adsorbs this type of polymer, the activities of colloid greatly decrease and the growth rate of the colloids in some certain facet will be confined [22,23]. Therefore, the addition of PEG in the metal oxide colloids will modify the growth kinetics of the growing colloids, which finally, leads to anisotropic growth of the crystals. For this reason, linear PEG has been widely used in the synthesis of a series of nanoparticulates and 1D materials in solution [22].
Polymeric materials decorated with inorganic nanoparticles are of particular interest because possible interactions between the inorganic nanoparticles and the polymer matrices may generate some unique physical properties upon the formation of various micro/nanocomposites [24][25][26][27]. Recently, interesting research has been focused on the polyaniline-inorganic nanocomposites to obtain the materials with synergetic or complementary behavior between polyaniline and inorganic nanoparticles [28][29][30][31].
Here, we report on the synthesis of PEG-Mn 3 O 4 nanocomposite. This is, so far, the first report using the adopted route for the synthesis of a PEG-Mn 3 O 4 nanocomposite. Results of comprehensive physicochemical and magnetic characterization are presented.

Chemicals and Instrumentation
All chemicals (Mn(acac) 2 , NH 3 , PEG-400, absolute ethanol) were analytical grade and were used without further purification.
X-ray powder diffraction (XRD) analysis was conducted on a Rigaku Smart Lab Diffractometer operated at 40 kV and 35 mA using Cu-Kα radiation (λ=1.54178Å).
Fourier transform infrared (FT-IR) spectra were recorded in transmission mode, in the range 4000∼400 cm −1 , with a Perkin Elmer BX FT-IR infrared spectrometer. The powder samples were ground with KBr and compressed into a pellet prior to analysis.
Transmission electron microscopy (TEM) analysis was performed using a JEOL JEM 2100 microscope.
A drop of diluted sample in alcohol was dripped on a carbon coated Cu TEM grid.
The thermal stability was determined by thermogravimetric analysis (TGA, Perkin Elmer Instruments model, STA 6000). The TGA thermograms were recorded for 5 mg of powder sample at a heating rate of 10℃ per min, in the temperature range of 30℃ ∼700℃ under nitrogen atmosphere. VSM measurements were performed by using a Quantum Design Vibrating Sample Magnetometer (QD-VSM). The sample was measured between ±15 kOe at room temperature. The electrical characterizations, AC conductivity and dielectric properties of PEG-Mn 3 O 4 nanocomposite were measured by using Novocontrol dielectric impedance analyzer in the frequency range of 1 Hz∼3 MHz. The samples were used in the form of circular pellets of 13 mm diameter and 3 mm thickness and the pellets were sandwiched between gold electrodes (two probe through plane). The temperature was controlled with a Novocontrol Cryosystem between 20 to 120℃. The dielectric data (ε and ε ) were collected during heating as a function of frequency.

Procedure
In a typical experiment, 1 g of Mn(acac) 2 was added dropwise into three-neck round-bottom flask. Then 16.7 ml PEG-400, heated and melted, was injected to the flask under NH 3 gas (pH=11). After continuous stirring, a homogeneous solution could be obtained. Then it was put in the autoclave and was kept at 160℃ for 12 h, then cooled to room temperature naturally. The products were filtered and washed several times with distilled water and absolute ethanol, and finally dried in a vacuum oven at 60℃ for 12 h.

XRD Analysis
XRD patterns of prepared samples are shown in

FTIR Analysis
In order to assess the interaction between the PEG and the nanoparticles, FTIR analysis of PEG as well as PEG-Mn 3 O 4 nanoparticles was performed; and the spectra are presented in Fig. 2. Vibrations of ions in the crystal lattice are usually observed in the range of 1000 ∼ 400 cm −1 in FTIR analysis. Two main broad metal-oxygen bands are seen in the FTIR spectra of all spinels, and ferrites in particular. The highest one, ν 1 , generally observed in the range 600∼500 cm −1 , corresponds to intrinsic stretching vibrations of the metal at the tetrahedral site (T d ), M tetra ↔O, whereas the ν 2 − lowest band usually observed in the range 450 ∼ 385 cm −1 , is assigned to octahedral-metal stretching Fig. 2(a)). Due to the limitation of our FTIR instrument below 400 cm −1 , ν 3 (328 cm −1 ) and ν 4 (below 300 cm −1 ) were not detected. The FTIR measurements reveal that the vibrational band of C-O bond shifts from 1113 cm −1 for pure ethylene glycol to 1095 cm −1 for the current Mn 3 O 4 4000 3500 3000 2500 2000 1500 1000 500 % Transmittance (a.u.) nanoparticles, which indicates that the O from C-O coordinates with metal on the surface of Mn 3 O 4 nanoparticles. Similarly, the bands around 2910 cm −1 and 955 cm −1 corresponded to −CH 2 stretching vibrations and −CH out of plane bending vibrations, respectively (as shown in Fig. 2(b)). The presence of −CH 2 and −CH peaks were strong evidence that PEG was chemically bonded to the surface. The surfactant molecules in the adsorbed state are influenced by the field of solid-state surface. As a result, the characteristic bands shifted to a lower frequency region (see Fig. 2(a)) [38,39].

Morphological Analysis
TEM micrographs of PEG-Mn 3 O 4 nanocomposite are shown in Fig. 3(a) and the particle size distribution obtained thereof is presented in Fig. 3(b). Particles are observed to have spherical morphology and seem to be aggregated probably due to the polymer coating. Size distribution histogram is obtained by measuring at least 150 nanoparticles and is fitted by using a lognormal function. An average size, D TEM/log−normal , of about 200 nm was obtained for magnetite nanoparticles. Crystallite size obtained from XRD line profile fitting is much smaller than the particle size estimated from TEM, revealing polycrystalline nature of Mn 3 O 4 nanoparticles. Polymer PEG coating on nanoparticles, however, is still not detectable via this imaging technique.

Thermal analysis TG analysis
Thermal stability of the precursor powder and final powder has been analyzed using TGA (see Fig. 4). To further confirm the existence of PEG on the surface of Mn 3 O 4 nanoparticles and quantify the proportion of organic and inorganic phase, TGA was performed in the temperature range of 30∼700℃. Pure PEG combustion started at about 340℃ and completely combusted at about 420℃ [40] (see Fig. 4(a)). Evidently, the combustion is delayed for the PEG capped Mn 3 O 4 nanoparticles, which start at about 380℃ and complete at about 410℃. The increase of combustion temperature is due to the extra interaction between the PEG and Mn 3 O 4 nanoparticles [41,42]. Product shows a major weight loss of about 20% over the temperature range of 30∼700℃ due to the decomposition and combustion of PEG (see Fig. 4(b)). This implies that product has almost 80% inorganic phase as Mn 3 O 4 nanoparticles.

DSC analysis
To observe the glass transition of PEG-Mn 3 O 4 nanocomposite, DSC analysis of the nanocomposite was performed and the obtained graph was given below (see Fig. 5). From DSC thermograph of nanocomposite, Tg of nanocomposite was found to be nearly 110℃ and this can explain significant increase in AC conductivity of the nanocomposite between 100℃ and 120℃ due to the secondary phase transition of nanocomposite as other reviewers have pointed out. We do not expect a breakdown mechanism.

Magnetization
Magnetic characterization of the sample has been performed at room temperature by measuring M (H) hysteresis curves up to magnetic field of ±15 kOe (see Fig. 6). It is observed that magnetization increases almost linearly with the field, i.e., paramagnetic-like behavior, and does not saturate even at the maximal applied field. Besides, sample exhibits small coercivity, which indicates that there is ferromagnetic ordering in magnetization of the samples. Therefore, total magnetization can be described as M(T, H)=χ P (T)H+M F , where χ P (T) is the paramagnetic susceptibility, H is the magnetic field and M F is the hightemperature ferromagnetic component of magnetization. M F was determined as approximately 0.3 emu/g from the intercept of the hysteretic part with the linear one in Fig. 6. If we consider the bulk magnetization of Mn 3 O 4 as 38 emu/g [3], we can deduce that sample has an impurity phase of nearly 0.8% from the ratio of saturation magne-tization values of ferromagnetic and paramagnetic parts. Mn 3 O 4 , which belongs to spinel oxides having the general form of AB 2 O 4 with Mn 2+ ions at A-sites and Mn 3+ ions at B-sites, exhibits three distinct magnetic transitions. The ferromagnetic ordering occurs at T N =42 K and at lower temperatures, Mn 3 O 4 undergoes a transition to an incommensurate phase at 40 K and a transition to a commensurate phase at 34 K [43]. Therefore, the hysteretic behavior observed in our sample can only be explained by the presence of small fraction of the impurity phase but not with the intrinsic magnetic interactions.

Temperature and frequency dependent conductivity and dielectric permittivity measurements
The frequency and temperature dependent AC conductivity and dielectric permittivity (dielectric constant and dielectric loss) properties of PEG-Mn 3 O 4 nanocomposite are studied over a broad frequency and temperature range. The related figures and explanations are given below.

AC conductivity
The AC conductivity of the PEG based nanocomposite including Mn 3 O 4 was measured from 20 to 120℃ using impedance spectroscopy as a function of frequency and temperature. The frequency-dependent AC conductivity graph is shown in Fig. 7. As well known, AC conductivity values were obtained using the following standard equation [44].  Where (ω) is the real part of conductivity, ω(2πf) is the angular frequency of the signal applied to the samples, ε is the imaginary part of complex dielectric permittivity and ε 0 (8.852 × 10 −14 F/cm) is the vacuum permittivity. The AC conductivity of nanocomposite including Mn 3 O 4 nanoparticles showed two different behaviors depending on both temperature and frequency. Regarding the temperature dependency of nanocomposite, the conductivity values were very low and in the range of 10 −15 to 10 −12 S/cm up to 100℃. Beyond this temperature, the conductivity has remarkably increased (more than 10 4 times) and reached to the level of conductivity of 10 −7 S/cm. It can be seen from Fig. 7 that the AC conductivities of PEG-Mn 3 O 4 nanocomposite at 1 kHz were 1.25×10 −15 S/cm and 3.67×10 −7 S/cm for 20 and 120℃, respectively, which show the conductivity increases with temperature. This significant improvement in conductivity with temperature can be directly attributed to the secondary phase transition of nanocomposite. Beyond this temperature, Mn 3 O 4 particles may interact with each other leading to a percolated path, which will facilitate the ionic conduction. This result shows that electrical current flows through both the semiconducting Mn 3 O 4 particles and dielectric PEG with the temperature range applied as reported earlier [44,50].
Concerning frequency dependence of AC conductivity, PEG-Mn 3 O 4 nanocomposite exhibits a frequency-dependent conductivity at lower temperatures and frequencies. Interestingly it was ob-served that the conductivity values are nearly the same beyond 20 kHz, independent of temperature, while nanocomposite exhibited a temperaturedependent behavior at low frequencies. This phenomenon can be considered as a strong clue for ionic conductivity.

Frequency and temperature dependence of dielectric constant (ε )
The dielectric constant (ε ) of PEG-Mn 3 O 4 nanocomposite is given in Fig. 8. The dielectric constant of the nanocomposite decreases with increasing frequency when temperature is kept constant. Furthermore, these curves keep their shapes but slide up at higher temperatures. These are well in agreement with thermosets and crosslinked polymers formerly reported in the literature [45,46]. Dielectric constants of polymers, in general, are known to decrease gradually with increasing frequency and similar behavior was observed for PEG-Mn 3 O 4 nanocomposites prepared in this study. This behavior can be attributed to the frequency dependence of the polarization mechanisms. The dielectric constant depends upon the ability of the polarizable units in a polymer to orient fast enough to keep up with the oscillations of an alternating electric field. When frequency increases the orientational polarization decreases since the orientation of dipole moments need a longer time than electronic and ionic polarizations do. This causes the dielectric constant to decrease. Furthermore, the increase of (ε ) towards the low frequency region is also seen from Fig. 8. This may be attributed to the blocking of charge carriers at the electrodes [47]. For example, ε values of nanocomposite were found to be 1.12 and 1.35 for 1 MHz and 100 Hz, respectively. Figure 8 also shows the variation of the dielectric constant (ε ) depending on temperature interval of 20∼120℃ for the nanocomposite product. As known, the dielectric constant of polar polymers such as PEG increases with increasing temperature due to the molecular orientation and arrangement [48]. It is clearly emphasized in Fig. 8 that (ε ) of the Mn 3 O 4 nanocomposite increases with temperature due to the enhancement of interfaces between Mn 3 O 4 and nonconductive polymer matrix as reported in literature [49]. Another interesting result can be seen from Fig. 8, the dielectric constant of nanocomposite markedly increases beyond 100℃. This result can be explained by the secondary phase transition of nanocomposite. At this temperature, chemically bonded PEG based naocomposite behaves as rubber-like and Mn 3 O 4 particles may easily interact with each other yielding a percolated path that will facilitate the conduction. Hence, this can cause a significant increase in dielectric constant above 100℃. The ε value of nanocomposites was found to be 24.4 at 100 Hz and 120℃ while the dielectric constant was 1.35 at the same frequency and 100℃. Frequency and temperature dependence of dielectric loss (ε ) Figure 9 shows the dielectric loss (ε ) variations dependent on the frequency applied at different temperatures for PEG-Mn 3 O 4 nanocomposites. It was observed that (ε ) increases as the frequency increases, reaches a maximum and thereafter decreases at 20∼100℃ temperature interval for nanocomposite. As the temperature increases, the frequency at which (ε ) reaches a maximum shifted towards higher frequencies. The maximum peak was observed at 1.4 kHz for 20℃ while the maximum was detected at 23.2 kHz for 90℃. On the other hand, no shifting curve was found for the samples measured at 110 and 120℃. This is probably due to the being out of frequency range applied in the study. Conclusively, as the temperature increase, the frequency at which ε reaches a maximum shifted towards higher frequencies. In addition, the activation energy for the observed relaxation process was evaluated from the following equation.
Where ω max is the frequency at which ε (tan dissipation) is maximum, k B is Boltzmann's constant (eV · K −1 ), T is temperature in Kelvin degrees and ΔE is the activation energy. The plot of log (ω max ) vs. 1/T is shown in Fig. 10. From the slope of the straight line, one evaluated activation energy was found to be 0.172 eV. This value of activation energy less than 1 eV indicates the predominance of electronic polarization at present. It is believed that this behavior can be explained as follows. The dielectric permittivity of nanocomposite suggests that polymer segmental motions of PEG and electron hopping between Mn 2+ and Mn 3+ may be coupled in the sample at lower temperatures than 100℃. Similar results were reported from ferrite  containing nanocomposites in our recent articles [44,50].

Conclusion
We report on the synthesis of PEG-Mn 3 O 4 nanocomposite via a hydrothermal route. The crystalline phase, identified as Mn 3 O 4 , was shown to have a crystallite size of 12 ± 5 nm from Xray line profile fitting. Average particle size from TEM was obtained as 200 nm, which reveals polycrystalline character of Mn 3 O 4 nanoparticles. The interaction between PEG-400 and the Mn 3 O 4 nanoparticles was assessed to be via carbonyl groups coordination. Temperature independent AC conductivity of PEG-Mn 3 O 4 nanocomposite beyond 20 kHz provides a strong evidence of ionic conduction through the structure.
The conductivity and permittivity measurements strongly depend on the secondary thermal transition of nanocomposite beyond 100℃. Above that temperature, Mn 3 O 4 particles may interact with each other yielding a percolated path that will facilitate the conduction. As the temperature increases, the frequency at which (ε ) reaches a maximum shifted towards higher frequencies. The maximum peak was observed at 1.4 kHz for 20℃ while the maximum was detected at 23.2 kHz for 90℃. Besides, the relatively lower activation energy (E a =0.172 eV) for relaxation process suggests that polymer segmental motions of PEG and electrons hopping between Mn 2+ and Mn 3+ may be coupled in the sample below 100℃.